Time-of-flight mass analyzers is a type of device that performs mass analyses by measuring the time of flight required for each ion to travel a specific distance and converting the time of flight to a mass. This analysis is based on the principle that ions accelerated by a specific amount of energy will fly at different speeds that correspond to their mass. Therefore, to improve the mass resolution, it is effective to provide the longest possible flight distance. For this purpose, multi-turn time-of-flight mass spectrometers have been developed and have successfully achieved high levels of mass resolution (for example, refer to Patent Documents 1 to 3 and Non-Patent Document 1). This type of mass spectrometer has a closed orbit of various forms (such as a substantially circular shape, substantially elliptical shape, “figure-8” shape or any other shapes), through which the ions are made to fly multiple times so as to increase their flight distance.
Another type of device developed for the same purpose is the multi-reflection time-of-flight mass analyzer, in which the aforementioned loop orbit is replaced by a reciprocative path in which a reflecting electric field is created to make ions fly back and forth multiple times. Although the multi-turn time-of-flight type and multi-reflection time-of-flight type use different ion optical systems, they are essentially based on the same principle for improving the mass resolution. Accordingly, in the context of the present description, the “multi-turn time-of-flight type” should be interpreted as inclusive of the “multi-reflection time-of-flight type.”
The multi-turn time-of-flight mass analyzer includes a multi-turn section in which ions are made to turn multiple times, an injector for injecting ions into the multi-turn section, and an ejector for extracting ions from the multi-turn section. The injector and ejector each have an ion-optical element that acts as a switch for creating a pulsed action to change the flight path of the ions, i.e. to deflect the ions or release them from their deflected state. These switches are hereinafter referred to as the injection switch and ejection switch, respectively. In most cases, the injection/ejection switch is realized by a deflecting electrode for changing the traveling direction of the ions. The injection switch can be used to control the mass range of the ions to be introduced into the multi-turn section. The ejection switch can be used to control the number of turns of the ions as well as other parameters.
As already stated, the multi-turn time-of-flight mass analyzer can achieve a high level of mass resolution. However, it has a drawback due to the fact that the flight path of the ions is a closed orbit. That is, the passing of ions: as the number of turns of the ions flying along the loop orbit increases, an ion having a smaller mass and flying at a higher speed passes another ion having a larger mass and flying at a lower speed. If the passing of ions having different masses occurs, it is possible that some of the peaks observed on the time-of-flight spectrum obtained by the measurement correspond to multiple ions that have completed different numbers of turns, i.e. those that have traveled different flight distances. In this case, it is impossible to uniquely relate the mass of the ions to their flight distance, so that the time-of-flight spectrum cannot be directly converted to a mass spectrum.
Taking into account this drawback, conventional multi-turn time-of-flight mass analyzers are normally used to realize a mass-zooming function intended for observing ions within a limited mass range where the passing of the ions produced by an ion source does not occur. This function is aimed at performing the measurement at a high mass resolution while limiting the target of observation to a relatively narrow mass range.
According to Non-Patent Document 1, the mass range, where no passing of the ions turning along the closed orbit occurs, is inversely proportional to the number of turns, so that the mass resolution and mass range of the measurement are also inversely proportional to each other. For example, if the ions are made to turn approximately one hundred times, the mass range where the passing of ions never occurs is reduced to a few percent as compared to the case where the ions are not made to turn. Therefore, if the sample requires high mass resolution and one must obtain a mass spectrum over a broad mass range for this sample, it is inevitable to perform a mass analysis while shifting the mass range for every analysis to obtain mass spectrums each covering a different mass range, and to eventually synthesize those mass spectrums to create a mass spectrum covering a broader mass range. Such a measurement requires a considerable length of time and seriously deteriorates the measurement throughput.
A method for expanding the mass range to be observed in the multi-turn time-of-flight mass analyzer is disclosed in Patent Document 4. According to this method, a multiple correlation function of a plurality of time-of-flight spectrums corresponding to different periods of time of ejection from the multi-turn section is calculated to reconstruct a single-turn time-of-flight spectrum from those time-of-flight spectrums. However, if there is only a small number of time-of-flight spectrums to be combined, this method may artificially create a false peak that does not really exist. Therefore, it is desirable to perform a mass analysis three or more times to obtain a plurality of time-of-flight spectrums for different periods of time of ejection from the multi-turn section. Thus, a measurement by this method also inevitably requires a long period of time. Furthermore, this method is also inefficient in that the calculation of the multiple correlation function generally involves complex operations that consume a considerable amount of time.
Patent Document 1: Japanese Unexamined Patent Application Publication No. H11-135060
Patent Document 2: Japanese Unexamined Patent Application Publication No. H11-135061
Patent Document 3: Japanese Unexamined Patent Application Publication No. H11-195398
Patent Document 4: Japanese Unexamined Patent Application Publication No. 2005-79049
Non-Patent Document 1: M. Toyoda et al,. “Multi-turn time-of-flight mass spectrometers with electrostatic sectors”, J. Mass Spectrom., 38, pp.1125-1142, 2003